Recent increase of mobile data usage and emergence of new applications such as gaming, mobile TV and streaming content have motivated the 3rd Generation Partnership Project (3GPP) to work on the Long-Term Evolution (LTE) in order to ensure 3GPP's competitive edge over other, competitive cellular technologies.
LTE has been set aggressive performance requirements which rely on physical layer technologies, such as e.g. Orthogonal Frequency Division Multiplexing (OFDM) and Multiple-Input Multiple-Output (MIMO) systems to achieve these targets. Some main objectives of LTE are to minimize the system and User Equipment (UE) complexities, to allow flexible spectrum deployment in existing or new frequency spectrum and to enable co-existence with other 3GPP Radio Access Technologies (RATS).
The LTE uplink is based on single-carrier modulation and uses frequency and time division multiple access principles (FDMA and TDMA), The LTE uplink consists of physical uplink control channels and data channels that are orthogonally frequency multiplexed. The single-carrier property of the LTE uplink makes it impossible for a UE to transmit on a physical control channel and a physical data channel in the same transmission-time-interval (TTI). Hence, if a UE is transmitting data on a physical data channel, the control information that has to be sent in the same TTI must also be sent on the physical data channel. The UE will use the physical control channel to transmit control signaling only in the case when the UE has no data transmission, and hence is not using the physical data channel.
In the LTE concept defined in the ongoing 3GPP work on standardization, the downlink will support fast channel dependent scheduling in both the time and frequency domains. A conventional downlink scheduling concept according to the prior art, can be described with stages 1:1-1:4, as illustrated in FIG. 1. A base station 100, which is referred to as an eNodeB in LTE, communicating with a UE 101, transmits reference signals to UE 101 in a first stage 1:1. The reference signals can be used by UE 101 to determine the present downlink channel quality. LTE uses feedback from the UE to the eNodeB of the instantaneous channel conditions. After having determined the downlink channel quality on the basis of the received reference signals, UE 101 therefore sends one or more channel state feedback reports, which in this context typically are referred to as Channel Quality Indication (CQI) reports, back to eNodeB 100 in a second stage 1:2. In eNodeB 100, the content of the one or more CQI reports can be retrieved and used by a scheduler (not shown). The scheduler uses the information retrieved from the CQI reports to perform resource allocation. UE 101 is informed of the resource allocation in a next stage 1:3. A resource allocation typically results in a transmission of downlink data over the allocated resource, as indicated with a final stage 1:4.
More on this issue can be found in “3G Evolution: HSPD and LTE for Mobile Broadband” E. Dahlman, S. Parkvall, J. Sköld, P, Beming, Academic Press, 2007.
According to one proposal for LTE, UEs will be capable of transmitting different types of CQI reports, such as full CQI reports, partial CQI reports, and/or differential CQI reports. In this context, full CQI reports are defined to cover the whole downlink transmission bandwidth scheduled for a UE, but may have different frequency resolution. This type of CQI reports may be filtered and processed in different ways. In addition, different full CQI reports may be encoded in different ways. Partial CQI reports on the other hand may be set to cover only a part of a specified downlink transmission bandwidth. The part covered by a partial CQI report may be a set of contiguous, or a set of distributed resource blocks. Finally, a differential CQI report may contain an encoded version of the update vector relative to a previous CQI report.
A CQI reporting mechanism which is based on different types of CQI reports, such as e.g. the ones described above, may be introduced by way of, for each CQI report type, defining a set of rules that triggers the transmission of a report of the respective CQI report type from a respective UE. Each UE has a configured CQI reporting trigger set (CRTS), wherein the CRTS consists of one or more CQI reporting triggers (CRT), specifying under which criteria a specific type of CQI report shall be transmitted. Each CRT is associated with a specific type of CQI report in such a way that when a triggering criteria is fulfilled, the respective UE transmits a CQI report of the associated type to the respective eNodeB.
A CRT is typically expressed in terms of a logical expression which may involve one of, or a combination of timers, events, and conditions, consisting of logical statements, such as AND, OR, NOT, WHEN, and/or IF. A simple periodic CQI reporting trigger may just consist of a periodic timer and a rule that a certain CQI report shall be transmitted every time the timer expires. A simple event based CQI reporting trigger may state that a certain type of CQI report shall be transmitted every time the triggering event, such as e.g. a handover event, occurs. A condition that could be included in the decision to transmit a certain CQI report or not, is e.g. if the downlink activity is above a specified threshold. In addition to configuring rules, defining when and how CQI reports are to be transmitted, the eNodeB may also have the option to explicitly request for CQI reports on demand, typically by using RRC signaling.
FIG. 2 illustrates a table of a CQI trigger configuration of a UE, as described above. The table comprises a plurality of CQI reporting triggers, CRT 1-n, configured for the UE. Each CRT is associated with one CQI report type, CQI A-X. When for example the trigger criteria specified by CRT 1 is true, a report type, defined by CQI A will be transmitted from the UE to an eNodeB, as indicated in the table. What types of CQI reports a UE shall use, and what criteria that will trigger them, are typically set-up by higher layer, RRC signaling.
CQI reports may occur at known time instances and may use a format known to the eNodeB, or the occurrence and format may be more dynamic. In the latter case the MAC header typically needs to include information about how the CQI report was transmitted, or else the eNodeB may have to perform blind detection on the CQI transmission format.
Furthermore, for CQI reports used together with different antenna configurations, such as SISO (Single-Input Single-Output), MISO (Multiple-Input Single-Output), SIMO (Single-Input Multiple-Output), or MIMO (Multiple-Input Multiple-Output), transmission could also be different. In case of a MIMO configuration, a CQI report may include information, such as e.g. pre-coding weights or other feedback parameters, to be used by the eNodeB multiple antenna transmission scheme. The amount of resources needed to be reserved for a certain UE will also depend on the MIMO scheme configured for that UE, potentially adding further complication to the configuration of the reserved resources.
From an overhead perspective, it is desirable to keep the number of bits in the CQI reports to a minimum. At the same time, the larger the number of bits in the CQI report, the higher amount of information can be provided to the scheduler of the eNodeB, allowing for the possibility of higher downlink throughput. Therefore, a trade-off between the two is required. The amount of bits that can be spent on CQI reporting may depend on a number of different criteria, such as: downlink transmission mode, e.g. SISO or MIMO; type of downlink traffic, e.g. VoIP or Web; downlink radio characteristics, e.g. coherence time and/or coherence bandwidth; current uplink load and/or current downlink activity.
CQI reports can be transmitted in two ways. A CQI report can be transmitted on a dedicated control channel resource when no data is transmitted simultaneously, or on a scheduled resource on a shared channel when uplink data and control signaling is transmitted simultaneously. A drawback with such a scheme is that resources must be reserved for control signaling; resources that will be unused when the UE is transmitting data simultaneously with control signaling. This further adds to the importance of keeping the CQI reporting overhead at a minimum.
Every UE normally have access to a number of radio bearers. To each radio bearer there is a QoS label specified, characterizing QoS requirements and traffic characteristics of the respective radio bearer. Some of these radio bearers are classified as Guaranteed Bit Rate (GBR) bearers, typically to be used for e.g. voice telephony or streaming video, while other radio bearers are classified as Non Guaranteed Bit Rate bearers.
In order to fully support all possible CQI reporting schemes in all possible scenarios one would have to allocate an unreasonable amount of physical resources for the physical control signaling.
Even with a limited number of schemes applied, new feedback schemes are difficult to introduce, especially if they require that the uplink physical control channels need to be re-designed.